Field of the Invention
The present invention relates to a friction plate used for a clutch and a brake of automatic transmission of vehicle.
Description of the Related Art
Drag torque between a friction plate and a counterpart plate during idling in the wet multiple-plate clutch of automatic transmission is a part of driving resistance associated with fuel economy of vehicle and reduction thereof greatly contributes to fuel economy improvement and CO2 emission control of vehicle. Conventionally, an effect for reducing the drag torque between the friction plates and the counterpart plates has been obtained by an effect for discharging (separating and the like) oil through a groove on a friction surface of the friction plate or an oil path between friction materials. This is disclosed in Japanese Patent Application Laid-Open No. 2005-076759 and in Japanese Patent Application Laid-Open No. 2008-180314.
Further, regarding a problem of demand for reducing the drag torque on the friction plates, how viscous resistance of fluid lubrication can be reduced specifically in a low rotation region is a key to solve the problem. In this regard, as disclosed in Japanese Patent Application No. 2014-149484, a certain effect can be obtained at 1000 rpm or less of a relative number of rotations by providing a groove on a segment-shaped surface of the friction plate.
However, fuel economy regulation and emission regulation in vehicle become stricter year by year. And if specifically a current problem of reducing viscous resistance of fluid lubricant that is high in a low rotation region that is around 500 rpm of a relative number of rotations in which the drag torque on the friction plates is great, is solved, a friction plate of wet multiple-plate clutch for automatic transmission can contribute toward achieving the fuel economy regulation and the emission regulation in vehicle. However, currently (in each of the above-mentioned references), there has not yet been found a method for reducing the viscous resistance of fluid lubricant of the friction plates in the low rotation region.
Further, a separation effect between friction plates using oil discharge force and an effect for reducing the viscous resistance of fluid lubricant by improving discharge property of oil by a groove of the friction plate, both of which effects are intended to reduce the drag torque, can be obtained by increase of rotation so that those effects have not yet been obtained in the low rotation region.